1. Field of the Invention
This invention relates to Read Only Memory (ROM) manufacturing techniques and devices produced thereby and more particularly to code implanting during ROM manufacturing.
2. Description of Related Art
ROM devices are standard components of modern computer systems. A ROM comprises an array of Metal Oxide Semiconductor Field Effect Transistor (MOSFET's) arranged in columns and rows, wherein predetermined MOSFET's are either permanently conductive or nonconductive as a function of the variety of transistor. The alternative on/off operation of these devices states of the MOSFETs is adapted to use for storage of data, which remains in the device when the external power supply is off.
A ROM device includes an array of parallel, closely spaced line regions formed of a heavily doped impurity in a semiconductor substrate having an opposite type of background impurity. On the surface of the substrate an insulating layer is formed thereon. Another array of closely spaced conductive lines formed on the surface of the insulating layer is arranged at right angles to the spaced lines in the substrate. Insulating layers are formed on the upper array of conductive lines. A metallurgy layer connects the two arrays of lines to circuits to address the lines and to read the data stored in the RAM, as is well known in the art.
At the intersection of a conductive line in the upper array which is commonly referred to in most cases as a "word line" and a pair of adjacent lines in the substrate, known in most cases as the "bit lines", a MOSFET is formed. The spaced lines in the substrate comprise the source and drain of the MOSFET. The conductive word line serves as the gate electrode of the MOSFET. Certain predetermined MOSFET's can be made permanently conductive by forming a region of an impurity of the same type as that in the bit lines, between adjacent bit lines and beneath the corresponding conductive line. These permanently conductive regions are known as code implants, and they are placed in the substrate to provide specific binary data.
The conventional manufacturing process is to form the code implant regions very early in the ROM fabrication process, since an annealing step is required to activate the implanted impurity and also to recrystallize any implanted areas of the substrate. The annealing process involves heating the substrate above an acceptable temperature, which would damage the completed device, since the aluminum metallization conventionally used is damaged above a temperature of about 400.degree. C. to about 450.degree. C. for more than a minimum time interval, and the maximum possible temperature is the melting point of aluminum which is about 660.degree. C.
An object of this invention is to form a ROM in which the damage caused by annealing to activate an implanted impurity is not employed subsequent to programming the code in the ROM.
A significant advantage of the process of this invention is that no implantation process is employed for programing following a metal etching step, so no thermal anneal is required.
Another advantage of the present invention is that the mask ROM process is suitable for VLSI applications and employs post metallization programing with no high temperature process requirement after programming.
The programming method of this invention employs an etching process to remove the channel area to encode the ROM, thus the transistor (memory cell) is open or non-conductive in the area removed.
For the structure of the instant invention, the gate electrode can also be made with polysilicon 1 and the source/drain and channel of the transistor can be formed at polysilicon 2 if a double polysilicon process is used.